Companions are front runners among all troops, with a great deal of hitpoints, they can endure more than any other troop. For their strength in taking damage, they do sheer damage themselves and cannot win battles unless they are supported by stronger troops.
As a Greek unique unit, the Companion has more 10% more health than standard heavy cavalry and 25% more damage.
The Companion is one of the strongest units in the game; as well as the strongest heavy cavalry.
"The Companions were the elite cavalry of the Macedonian army from the time of King Philip II of Macedon and achieved their greatest prestige under Alexander the Great, and have been regarded as the best cavalry in the ancient world and the first shock cavalry. Chosen Companions/Hetairoi formed the elite guard of the king (Somatophylakes)."
Companions have a lot of health but are slow and have low attack power, so use them in groups.
Companions are weak against many defenses grouped together.
Due to their high health, companions can be use as meat shields to protect infantry from the defenses.
When pushing for trophies, you would need to destroy the Town Center. In many bases, the town center would be in the center of the base. First, train up an army of companions. When attacking, deploy all your companions in the areas which the Town Center is more close to. If there are a few defenses blocking the way, destroy them. Then rally your companions to destroy the Town Center.
Companions would ignore defensive troops and would pursue their targets which are defensive buildings. However, if a defensive troop attacks the companion, the companion will attack back until the defensive troop is killed; thus making the companion distracted. Try to use and deploy ranged infantry such as composite bowmen or heavy infantry such as hoplites after deploying the companions to eliminate the defensive troops.
Companions are a good alliance troop due to its high health; making it longer to kill them. However, they take up 6 space in the alliance gate so it cannot be used in a level 1 Alliance Gate in the Iron Age (which can only hold 3 spaces) until it is upgraded to level 2 which is available in the Classical Age.
A misconception in the photo above is that the companion seems to be wearing a Galea helmet (a Roman helmet) and is shirtless. Companions would wear Boeotian helmets instead since it allowed good peripheral vision and unimpaired hearing. Companions would also wear a bronze muscle cuirass, linothorax (linen upper body armor), and shoulder guards. If a companion goes to war shirtless, horsemen with javelins or spears could pierce into the companion easily, and if archers shoot their arrows, the arrows would go through the companion easily killing him. Another misconception is that companions don't use shields. The shield also has the lambda sign on it which represents Sparta. Sparta would never used cavalry such as companions and would rely on infantry such as hoplites and phalanxes.
Companions uses 2 weapons; the xyston (a spear, the main weapon of the companion) and a kopis (curved slashing sword) or xiphos (cut and thrust sword) which is used for close combat and a replacement if the xyston breaks. The image shows the companion holding a xiphos.